Skin packaging can be classified as a vacuum forming process for thermoformable polymeric films. However, the term "vacuum skin packaging" or VSP as it is referred to hereinafter, refers not to the fact that the thermoformable film is formed around the product by vacuum or differential air pressure which, indeed it is, but more to the fact that the product is packaged under vacuum and the space containing the product is evacuated. For a number of products there is a need for the film formed around each product to be a barrier to oxygen, air, and other gases.
In conventional skin packaging, a backing board which is porous or which is perforated is used so that a vacuum may be drawn directly through the backing board. In vacuum skin packaging processes, generally, a vacuum chamber with an open top is used. A product on an impervious backing board is placed on a platform within the vacuum chamber. The opening to the upper chamber is covered by a sheet of film which is clamped tightly against the chamber to form a vacuum tight closure. The chamber is evacuated while the film is heated to its forming and softening temperature. The platform is then raised to drive the product into the softened film and air pressure is used above the film to force it tightly around the product. A similar type process is disclosed in French Patent No. 1,258,357 which issued to Alain G. Bresson on Mar. 6, 1961.
In another version of vacuum skin packaging in the prior art, which is disclosed in U.S. Pat. No. 3,491,504, which issued to W. E. Young et al on Jan. 27, 1970, heat softened film is physically moved down over a stationary product and, with the use of differential air pressure, the softened thermoplastic film is molded onto the product.
In U.S. Pat. No. Re. 30,009, which was reissued on May 29, 1979 to Richard R. Perdue et al, a thermoformable or heat softenable film sheet is drawn by differential air pressure against the concave interior surface of the upper portion of a vacuum chamber, the film is then heated by surface contact, and then, after evacuation of the chamber, air pressure is used to blow the film down over the product and against the backing board. The resulting package comprises the product positioned on the backing board which is gas impervious. The product is held on the backing board by the thermoformable film which has been formed around the product in the exact shape of the product so that it appears to be an almost invisible "skin." The thermoformable film or forming web, as stated previously, is also gas impervious and usually will consist of a number of layers each of which performs a specific function. The layer in contact with the product and backing member will be a sealing or heat sealable layer. An interior layer will typically be a barrier layer which comprises a vinylidene chloride copolymer or a hydrolyzed ethylene/vinyl-acetate copolymer, and the outer surface layer will be an abuse layer to protect the barrier layer from scratches, pin holes, or from moisture attack.
In U.S. Pat. No. 3,574,642 which issued on Apr. 13, 1971 to Carl Frederick Weinke, a package for a method of packaging meats is disclosed. The package includes an inner oxygen-permeable member which may be also gas flushed or evacuated. The package preserves the freshness of the meat until the meat is ready to be marketed to the consumer. For marketing, the outer wrapper is removed and the inner package is displayed to the consumer. Being oxygen-permeable, the inner wrapper admits oxygen to the interior of the package causing the fresh meat product to change to a bright red color which the consumer associates with freshness. The inner pouch of the Weinke package may consist of polyethylene film and the outer pouch may be cellophane film with a coating of saran (vinylidene chloride copolymer.) Another patent showing portions of fresh meat individually packaged in oxygen permeable plastic film and inserted into an outer container of impermeable film is U.S. Pat. No. 3,681,092 which issued to Oliver R. Titchnell et al on Aug. 1, 1972.
Another prior art package is described in U.S. Pat. No. 3,713,849 which issued to Paul E. Grindrod et al on Jan. 30, 1973. In the Grindrod et al patent a fresh meat package having an outer oxygen impermeable lamina which is readily and entirely peelable from an inner oxygen-permeable lamina is disclosed. The package includes means for initiating the peeling separation along an edge of the package. The outer oxygen barrier maintains meats in well preserved condition in spite of the purplish color which has low consumer appeal. Shortly prior to display for sale to the consumer the outer lamina is removed by the retailer and the product develops a healthy, bright red "bloom" due to the high oxygen through-put of the inner remaining film package. The material disclosed in Grindrod et al is a laminate of PVC/Saran and EVA/Saran. (EVA designates ethylene/vinyl-acetate copolymer and PVC designates polyvinyl-chloride.) The EVA and PVC layers are the inner layers and at the periphery of the package they are sealed together. The saran layers can be readily peeled from the respective EVA or PVC layers with gripping tabs that are provided.
Yet another peelable package is shown in U.S. Pat. No. 4,055,672 which issued on Oct. 25, 1977 to Arthur Hirsch et al. In the Hirsch et al patent a semi-rigid preformed tray of oxygen impermeable material is formed, a metal product placed therein, and then the tray is sealed around its upper periphery or flange area by a composite lid which has an inner layer of oxygen impermeable material, an adhesive layer, and an outer layer of oxygen impermeable material. When the package is ready for retail display, in order that oxygen can reach the fresh meat packaged within impermeable material, the outer, impermeable lid is peeled away so that the oxygen can penetrate through the remaining portion of the lid. Accordingly, another object of the present invention is to provide a package with a strippable or peelable barrier layer which is an improvement over prior art packages.
In a number of instances in order to preserve a meat product within an impermeable film, it is desirable to provide a modified atmosphere which may include inert gases such as nitrogen or the like or an atmosphere which has a mixture of gases such as oxygen and carbon dioxide to control the bacterial growth within a package. Typical packages are disclosed in U.S. Pat. No. Re. 27,872 which issued on Jan. 8, 1974 to J. J. Estes; U.S. Pat. No. 2,623,826 which issued on Dec. 30, 1952 to Sanford R. Grinstead; U.S. Pat. No. 3,360,382 was issued on Dec. 26, 1967 to H. V. Miller; U.S. Pat. No. 4,522,835 which issued on June 11, 1985 to Richard E. Woodriff; U. K. Patent No. 1,199,998 which was published July 22, 1970; Canadian Patent No. 890,766 which issued Jan. 18, 1972 to Charles M. Davison et al; U. K. Patent No. 1,378,140 which was published Dec. 18, 1974; and in U. K. Patent No. 1,186,978 which was published Apr. 8, 1970. U.S. Pat. No. 2,925,346 which issued Feb. 16, 1960 discloses a process of packaging cured meat products and inhibiting color degradation by removing oxygen and back filling the container with a gaseous oxide of nitrogen. Accordingly, it is another object of the present invention to provide a vacuum skin package which is suitable for and can be used with modified atmospheres.
In order to readily open packages where plastic film layers have been sealed together to close the package, various tear tabs and easy open mechanisms have been devised. One such easy-open, delaminating seal is disclosed in U.S. Pat. No. 4,638,913 which issued on Jan. 27, 1987 to Milton A. Howe, Jr. In this patent, two grippable film folds are provided to act as gripping tabs and the folds, when pulled apart, will rupture one of the outer layers of the sealed together film and delaminate the film to its edge. In such a case, of course, the bond strength between the two sealed together films must be greater than the layer-to-layer bond strength of the film. Accordingly, it is still another object of the invention to provide a package which it is readily openable.
The foregoing and other objects are achieved by the present invention which is described in the Summary of Invention below, and an embodiment of which is shown in the attached drawings, and which is further described in the detailed description. The advantages of the invention will become apparent to those skilled in the art upon reading this disclosure which is understood not to be limited to the embodiments described herein.